The claimed invention relates to multicarrier communications and, more particularly, to adaptive modulation in multicarrier communications.
Multicarrier communications may be described as a communications technique in which multiple carriers or subcarriers are used to communicate information. As an example of multicarrier communications, Orthogonal Frequency Division Multiplexing (OFDM) may be described as a communications technique that divides a communications channel into a number of spaced frequency bands. In OFDM, a subcarrier carrying a portion of the user information may be transmitted in each band. In OFDM, each subcarrier may be orthogonal, differentiating OFDM from the commonly used frequency division multiplexing (FDM). An OFDM symbol may include, for example, a multicarrier symbol transmitted simultaneously on the OFDM subcarriers during the OFDM symbol period. The individual symbols on particular subcarriers may be referred to as subcarrier symbols.
In wireless systems that are susceptible to dynamic or frequency selective fading, such systems may benefit substantially from channel interleavers that interleave at the transmission bit level to break up fading that may be correlated within a transmission symbol. Some OFDM communication systems may also use Adaptive Bit Loading (ABL), which may also be referred to as adaptive subcarrier modulation, where the modulation density may be changed on a subcarrier-by-subcarrier basis based on channel conditions. Techniques such as ABL may complicate the bit interleaving process, however, because each OFDM symbol may contain a different number of bits whenever a new channel adaptation is made (e.g., transmission). It may be difficult to implement an efficient interleaving scheme when the number of bits may vary among OFDM symbols.